1. Field of the Invention
The present invention relates to an MRI (magnetic resonance imaging) apparatus which generates a uniform static magnetic field and gradient magnetic fields.
2. Description of the Related Art
A medical imaging apparatus provides a large amount of information on a patient in the form of an image, and plays an important role in many medical practices including a diagnosis of a disease, a treatment, and an operation planning. Currently, major medical imaging apparatuses include an ultrasonic diagnostic apparatus, an X-ray computerized tomography (CT) apparatus, an MRI apparatus, and a nuclear medicine diagnostic apparatus. In particular, the MRI apparatus can collect a high-quality contrast image of a soft tissue, and occupies an important place in the medical diagnostic imaging.
The MRI apparatus includes a cylindrically shaped static field magnet having an axis extending in an advance and retreat direction of a table-top, a cylindrically shaped shim coil having an axis extending in the advance and retreat direction of the table-top and provided inside the static field magnet, a cylindrically shaped gradient coil having an axis extending in the advance and retreat direction of the table-top and provided inside the shim coil, and a cylindrical liner having an axis extending in the advance and retreat direction of the table-top and provided inside the gradient coil to form a bore in which the table-top is advanced or retreated (see Japanese Patent Application Publication No. 2001-198102, for example). In an imaging process, the MRI apparatus generates a static magnetic field in a bore formed by the liner, and causes the gradient coil to form gradient magnetic fields in an X-axis direction, a Y-axis direction, and a Z-axis direction in a field of view for imaging a patient set inside the bore. Further, the MRI apparatus transmits a high-frequency signal from a radio frequency (RF) coil provided to thereto to magnetically resonate the nuclear spin in the patient and reconstruct an image of the patient with the use of a nuclear magnetic resonance (NMR) signal generated by the excitation.
The MRI apparatus normally requires a static magnetic field strength of approximately a few kilogauss to approximately ten kilogauss (one tesla). Further, the MRI apparatus is required to have a high spatial uniformity, i.e., a spatial uniformity of dozens of ppm or less. The spatial area required by the MRI apparatus frequently has a spherical size of 50 [cm] in diameter.
Along with an increase in the speed of the imaging technique, high-speed switching and increased strength of the gradient magnetic fields are indispensable in the MRI apparatuses of recent years.
Due to the indispensability of the high-speed switching and the increased strength of the gradient magnetic fields in the MRI apparatuses of recent years, the gradient coil is applied with a high current, and thus the amount of the heat generated by the gradient coil tends to increase. However, the generated heat cannot be sufficiently suppressed solely by a gradient coil cooling system present in the conventional technique. Due to the heat conduction and radiation, therefore, the heat generated by the gradient coil is conducted to an outer circumferential surface of the liner provided inside the gradient coil. Accordingly, an inner circumferential surface of the liner is also locally increased in temperature.
According to the conventional technique, an examination room housing the MRI apparatus is fully air-conditioned, and the bore formed by the liner includes therein a device having an air blowing function or the like. Such a device, however, cannot handle the local increase in temperature of the inner circumferential surface of the liner due to the heat generated by the gradient coil. In some cases, the heat generated by the gradient coil locally increases the temperature of the inner circumferential surface of the liner in contact with the patient up to approximately 50° C. As a result, the patient may feel discomfort from his contact with the liner. The contact of the patient with the liner even poses a risk of causing a low-temperature burn to the patient.
Further, in recent years, there has been a possibility of a reduction in thickness of the liner along with a tendency to enlarge the bore. In such a case, the heat of the outer circumferential surface of the liner is easily conducted to the inner circumferential surface of the liner. Consequently, the discomfort to the patient is increased, and the risk of the low-temperature burn is also increased.